Four-level pulse amplitude modulation (PAM4) is a higher-order modulation format whose spectrum efficiency is twice that of a non-return-to-zero (NRZ) code. In backplane interconnection, the PAM4 has been included into a study of IEEE 802.3bj. In next-generation 100G Ethernet, the PAM4 modulation format receives extensive attention.
As shown in FIG. 1, in an intensity-modulation and direct-detection (IMDD) system, a specific process of PAM4 modulation and demodulation includes: encoding, by a transmit end, two 2-level NRZ signals into one 4-level PAM signal by using a PAM4 encoder; converting, by a transmitter, the PAM signal into a PAM optical signal, optically modulating the PAM optical signal by using an electro-optic modulator, and sending an optical signal obtained through the optical modulation; and converting, by a receive end, the optical signal into an electrical signal by using a receiver, and converting the electrical signal into two 2-level NRZ signals by using a PAM4 decoder.
Specifically, if a PAM optical signal is denoted as S, and an optical carrier is denoted as C, the optical signal obtained by the transmitter after performing optical modulation is (C+S), and a process of converting the optical signal into the electrical signal by the receive end by using the receiver is: (C+S)(C*+S*)=CC*+(CS*+C*S)+SS*, where CC* means that a carrier is multiplied lied by a conjugate of the carrier, and that a direct current is generated and is filtered out by a DC blocking capacitor in the receiver; CS* and C*S respectively means that a carrier is multiplied by a conjugate of a signal and that a conjugate of the carrier is multiplied by the signal, and information that needs to be received is generated; and SS* is signal-signal beat interference (SSBI).
In the foregoing PAM4 modulation and demodulation process, the PAM optical signal is modulated at QUAD points (where callouts in FIG. 4 below may be referred to) on a modulation curve of the electro-optic modulator. That is, when optical modulation is performed, a bias voltage of the electro-optic modulator is set to H·Vpi+Vpi/2 (where H is a positive integer, a negative integer, or 0), so that SSBI is generated after the receiver converts the optical signal into the electrical signal. Because the SSBI interferes with a useful signal, the generated SSBI affects performance of the IMDD system. Vpi is a variation value of the bias voltage in any cycle of the modulation curve of the electro-optic modulator when an output optical power of the electro-optic modulator varies from a maximum value to a minimum value or varies from a minimum value to a maximum value, and the modulation curve is a curve corresponding to a function whose independent variable is the bias voltage and dependent variable is the output optical power. When the bias voltage is 0, the output optical power is the largest.